In an automatic control system, the control is utilized to modulate some system characteristic (called an independent variable) to achieve some desired other system characteristic (called a dependent variable). In other words, the automatic control system will modulate the independent variable to achieve a desired characteristic of the dependent variable. For example, if an automatic control modulates engine fuel flow to achieve a desired engine speed, then engine fuel flow is the system independent variable and engine speed is the dependent variable.
There often exists more than one independent variable capable of influencing any one dependent variable in a system. In such systems, it may be beneficial to use the multiple independent variables to achieve a desired characteristic of one dependent variable. The prior art typical automatic control includes an integral control which may comprise a simple integral control, or a proportional-plus-integral control, or a proportional-plus-integral-plus-derivative control. The integral portion of the control typically integrates the difference between the desired and actual dependent variable. The integrated result is used to modulate the independent variable. Thus, in concept, the one dependent variable desired characteristic could be achieved by simultaneously utilizing a combination of separate integral controls for modulation of each independent variable. However, it is well known in the prior art that simultaneously active multiple integral controls, each integrating the same dependent variable error, will cause a control problem, i.e., the multiple integrators, each modulating a separate independent variable, will fight each other in an attempt by each integrator to achieve a zero-error of the dependent variable. Therefore, it is an accepted practice in the prior art that any one dependent variable will not simultaneously utilize more than one integral in the automatic control, even though the automatic control may utilize various combinations of proportional and derivative control to modulate additional independent variables. This implies that the one integral control will modulate only one independent variable.